1. Field of the Invention
This invention relates to a technology of improving the quality of an amorphous conductor film constituting a solar cell.
Amorphous solar cells have already gained widespread practical acceptance as an energy-supplying source having a small output for driving electronic portable calculators or watches. However, for large outputs of 0.1 W or higher for solar light power generation applications, amorphous semiconductor films having satisfactory performance and stability have not been developed as an energy-supply source. Much work is being done for increased performance. This problem is essential to an amorphous hydrogenated silicon film, formed by such film-forming methods as a plasma CVD method, a photo CVD method, or a thermal CVD method, and a further improvement in performance was considered to be difficult.
For a solution to this problem, Japanese Laid-Open Patent Publication No. 14420/1988 disclose a process wherein the formation of a film and its plasma treatment with hydrogen or a halogenating substance is repeated. The improvement achieved by this technique has not been satisfactory. The present inventors considered that the main cause of this problem is that a large amount of bound hydrogen is involved during film formation to form an amorphous hydrogenated silicon. Furthermore, the present inventors considered the fact that most semiconductor films constituting amorphous solar cells now in practical use are formed by the plasma CVD method, and searched for a method which enables the plasma CVD method to be used in addition to other methods in a film-forming step.
As a result, the present inventors found that a semiconductor having a narrow optical band gap with a small amount of bound hydrogen can be obtained by repeating a step of improving the properties of the film, for example, comprising forming a semiconductor film having a specified amount of bound hydrogen to a specified thickness, and exposing the film to a discharge atmosphere containing a reactive gas, whereby the electro-optical properties of the resulting semiconductor film are much better than a conventional Si film.